It is not uncommon for operators of battery-powered portable communications devices, such as wireless telephones, personal data assistants, wireless pagers, and portable computers, to carry one or more spare batteries to extend the operational time of the device. External-type chargers are popular because they allow the operator to continue using the communications device while the other battery charges. However, typically portable chargers only allow a single battery to be charged at a time. Such “single-capacity” chargers have the obvious disadvantage of requiring lengthy recharge periods, particularly where the operator has multiple spare batteries. Therefore, attempts have been made to provide a battery charger that allows the operator to charge more than one battery at a time.
For instance, Murakami (U.S. Pat. No. 6,794,851) describes an external battery charger that allows a mobile phone to charge the battery that is installed in the mobile phone, while simultaneously charging a second battery that is external to the mobile phone. To apportion the charge current between the batteries, Murakami makes use of the fact that the resistance of a battery increases as the battery becomes charged.
Frame (U.S. Pat. No. 6,005,358) describes a portable computer docking station that supplies power to a portable computer, and includes a first charge circuit for charging the battery installed in the portable computer, and a second charge circuit for charging the battery installed in the docking station. Each charge circuit measures the current that is drawn from the power supply, and adjusts the charge current that they supply to their respective batteries based on these measurements.
None of these solutions make optimum use of the maximum charge current that is available from the power supply. As a result, charge times are unnecessarily long.